Floating guidance structures are intended to promote safe passage for juvenile salmon migrating downstream through reservoirs. However, the ability of an engineered structure to guide fish to safe passage has been primarily tested through large-scale implementation in reservoirs or in laboratory studies and computer simulations without live fish. Research is needed that integrates fluid mechanics with fish behaviour to study how hydraulic conditions around a guidance structure trigger swimming behaviours. In this study, an outdoor experimental channel was used to identify: (a) the hydraulic signature of a floating guidance structure and (b) changes in fish swim behaviour in relation to channel hydraulics. The flow field surrounding a guidance structure at two deployment angles was characterized using acoustic Doppler velocimeters. Swimming behaviours of juvenile Chinook salmon were recorded using underwater videogrammetry. A statistical method for behaviour change detection identified the most likely locations of swimming behaviour changes in fish as they first encountered the guidance structure. Finally, the locations of behaviour changes were compared to the hydraulics surrounding each guidance structure. Taken together, results indicated the fish did respond to the guide wall with behaviour changes, but did not distinguish between the two guide wall angles. While the two guide wall angles did produce statistically different distributions of hydraulic variables, the differences were small, potentially too small for the fish to produce a behavioural response. To inform the design of guidance structures, further work may clarify if swim responses vary with more aggressive wall angles and/or higher approach velocities, or if any contraction of flow and/or visual cue will produce similar behaviour responses. K E Y W O R D S environmental fluid mechanics, fish behaviour, fish passage, floating guidance structure, guide wall, hydraulics, reservoirs 1 | INTRODUCTION A smolt (seaward migrating juvenile salmonid) often has several routes for passing a dam, each with its own limitations. Of all passage routes at hydroelectric projects, turbines produce the highest rates of injury and mortality, which can be caused by high shear stress, turbulence, cavitation, decompression, blade strike, and mechanical wounding